Terephthalic acid and other aromatic carboxylic acids are widely used in manufacture of polyesters, commonly by reaction with ethylene glycol, higher alkylene glycols or combinations thereof, for conversion to fiber, film, containers, bottles and other packaging materials, and molded articles.
In commercial practice, aromatic carboxylic acids are commonly made by liquid phase oxidation in an aqueous acetic acid solvent of methyl-substituted benzene and naphthalene feedstocks, in which the positions of the methyl substituents correspond to the positions of carboxyl groups in the desired aromatic carboxylic acid product, with air or another source of oxygen, which is normally gaseous, in the presence of a bromine-promoted catalyst comprising cobalt and manganese. The oxidation is exothermic and yields aromatic carboxylic acid together with by-products, including partial or intermediate oxidation products of the aromatic feedstock, and acetic acid reaction products, such as methanol, methyl acetate, and methyl bromide. Water is also generated as a by-product. Aromatic carboxylic acid, typically accompanied by oxidation by-products of the feedstock are commonly formed dissolved or as suspended solids in the liquid phase reaction mixture and are commonly recovered by crystallization and solid-liquid separation techniques. The exothermic oxidation reaction is commonly conducted in a suitable reaction vessel at elevated temperature and pressure. A liquid phase reaction mixture is maintained in the vessel and a vapor phase formed as a result of the exothermic oxidation is evaporated from the liquid phase and removed from the reactor to control reaction temperature. The vapor phase comprises water vapor, vaporized acetic acid reaction solvent and small amounts of by-products of the oxidation, including both solvent and feedstock by-products. It usually also contains oxygen gas not consumed in oxidation, minor amounts of unreacted feedstock, carbon oxides and, when the oxygen source for the process is air or another oxygen-containing gaseous mixture, nitrogen, carbon oxides and other inert gaseous components of the source gas.
Pure forms of aromatic carboxylic acids are often favored for manufacture of polyesters for important applications, such as fibers and bottles, because impurities, such as by-products generated from aromatic feedstocks in such oxidation processes and, more generally, various carbonyl-substituted aromatic species are known to cause or correlate with color formation in polyesters made from the acids and, in turn, off-color in polyester converted products. Aromatic carboxylic acids with reduced levels of impurities can be made by further oxidizing crude products from liquid phase oxidation as described above at one or more, progressively lower temperatures and oxygen levels, and during crystallization to recover products of the oxidation, for conversion of feedstock partial oxidation products to the desired acid product, as known from U.S. Pat. Nos. 4,877,900, 4,772,748 and 4,286,101. Preferred pure forms of terephthalic acid and other aromatic carboxylic acids with lower impurities contents, such as purified terephthalic acid or “PTA”, are made by catalytically hydrogenating less pure forms of the acids, such as crude product comprising aromatic carboxylic acid and by-products generated by liquid phase oxidation of aromatic feedstock or so-called medium purity products, in solution at elevated temperature and pressure using a noble metal catalyst. In commercial practice, liquid phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid and purification of the crude product are often conducted in continuous integrated processes in which crude product from liquid phase oxidation is used as starting material for purification.
The high temperature and pressure vapor phase generated by liquid phase oxidation in such processes is a potentially valuable source of recoverable acetic acid reaction solvent, unreacted feed material and reaction by-products, as well as energy. Its substantial water content, high temperature and pressure and corrosive nature due to components such as gaseous methyl bromide, acetic acid solvent and water, however, pose technical and economic challenges to separating or recovering components for recycle and recovering its energy content. Further, impurities that remain unseparated in recovered process streams can prevent re-use of streams if impurities adversely affect other process aspects or product quality. As described in U.S. Pat. No. 5,200,557, for example, monocarboxylic acids adversely affect hydrogenation catalysts used in purification processes, with even low levels of acetic acid residues such as present in crude aromatic carboxylic acid products recovered from oxidation reaction liquids being considered detrimental.
British Patent Specification 1,373,230, U.S. Pat. Nos. 5,304,676; 5,723,656; 6,143,925; 6,504,051, European Patent Specification 0 498 591 B1 and International Application WO 97/27168 describe processes for manufacture of aromatic carboxylic acids by liquid phase oxidation of aromatic feed materials in which a high pressure off-gas is removed from oxidation and treated for recovery and recycle of portions or components thereof and, in some cases, recovery of energy. Condensation of off-gas, as in U.S. Pat. No. 5,304,676, is effective for recovery of water, acetic acid and other condensable components of the off-gas but separating water, acetic acid and other components in the resulting condensate is technically complex and economically impractical. High pressure off-gas separations, as in processes of U.S. Pat. Nos. 5,723,656, 6,143,925, 6,504,051 and WO 97/27168, can be effective for separating off-gases to recover acetic acid-rich liquids and gases comprising water vapor suitable for further processing. However, certain by-products of the oxidation tend to apportion into both liquid and gas phases in such separations, complicating their recovery and potentially adversely impacting other process streams and steps. These difficulties are compounded by build-up of such by-products in processes in which by-product-containing streams, such as mother liquor remaining after recovery of pure forms of aromatic carboxylic acid from a purification liquid reaction mixture or liquids condensed effluent gases from high pressure separations are used in separations. None of the processes according to the cited patents uses liquid condensed from a high pressure off-gas from a liquid phase oxidation as solvent or other liquid comprising water in the purification of impure aromatic carboxylic acids and recoveries of materials and energy in such processes often are accomplished at the expense of each other, for example due to loss of energy content on cooling or depressurizing to recover materials, burning of materials to control atmospheric emissions and other losses of oxidation solvent, feedstock and by-products that result if a high temperature and pressure vapor phase from oxidation is not cooled or depressurized for removal of such materials.
Impurities remaining in recycle streams can upset process operation and impair product quality. Added equipment and process steps for recovering materials, energy or both can add further process complexities and limit or preclude their practical utility if they add costs that outweigh materials and energy savings. Impact of such factors, lost energy and lost materials are magnified by scale of process operations. In world-scale commercial manufacturing plants with annual capacities of 500,000 to 1,000,000 or more tons of product, even fractional percentages or hundreds of parts per million of feedstock and solvent lost or converted to undesired or unusable by-products, minor inefficiencies in energy recovery and incremental additions to effluent water treatment translate to significant practical losses of materials, increases in consumption of fuel or electricity and added processing, as well as unpredictable process efficiencies and economics due to differences and variations in costs for energy, materials and requirements for treatment of gaseous and liquid emissions and effluents.